1. Field of the Invention
The present invention relates to a polishing amount control system for controlling the amount of polishing with respect to the coating layer of a magnetic disk or other workpiece and a method for the same.
2. Description of the Related Art
In general, nickel is a ferromagnetic metal, but becomes nonmagnetic when made an amorphous plating film and incorporating a suitable amount of phosphorus. As one example, there is a nonelectrolytic nickel-phosphorus plating film. A plating film of this nonelectrolytic nickel-phosphorus (hereinafter referred to as "Ni-P") has a high dynamic strength, high corrosion resistance, and, as mentioned above, nonmagnetic property, so as to be suitable for use as a reinforcement film for a magnetic disk etc.
That is, the practice has been to form a layer of a nonelectrolytic Ni-P plating film on both surfaces of a magnetic disk substrate, polish the surface of the plating layer to flatten it, and thereby form a predetermined magnetic disk.
A plating layer of a magnetic disk, however, may be given flatness by polishing to a predetermined thickness. Further, even a plating layer is polished flat, if the thickness of the plating layer on one surface of the magnetic disk differs from the thickness of the plating layer on the other surface, there is liable to be a difference in the properties of the front side of the magnetic disk and the properties of the reverse side of the magnetic disk.
Therefore, in the past, the thicknesses of the plating layers of polished magnetic disks have been measured by an X-ray thickness meter to investigate if the plating layer has been polished to a desired thickness and the difference between the thickness of the plating surface of one surface and the thickness of the plating surface of the other surface has become within a predetermined range. Further, when the polishing conditions change due to roughening of the polishing pads etc. and the plating layers can no longer be polished to the desired thickness, the rotational speeds of the platens, sun gear, etc. of the polishing apparatus have been changed manually to control the amount of polishing of the plating layers.
There were however the following problems with the above method of control of the polishing amount of the related art.
In general, magnetic disks are polished as part of the flow of work. Magnetic disks continue to be polished even when measuring the thicknesses of the plating layers of the polished magnetic disks. Therefore, in the interval between when the thicknesses are measured to when the rotational speeds of the platens etc. of the polishing apparatus are controlled, poorly polished magnetic disks end up being produced.
In practice, the thicknesses are often measured for one or two magnetic disks sampled in batch units (for example, 50 magnetic disks). In some cases, further, the thicknesses are measured only once half a day. In such a case, poorly polished magnetic disks end up being produced in large quantities until the upper platen and lower platen of the polishing apparatus are controlled to a suitable rotational speed.
As opposed to this, it may be considered to wait until the end of the series of work of measuring the plating layers and deciding on the rotational speeds of the upper platen and lower platen before polishing the next work. However, since this work is performed manually, a long time would be required to feed back the results of the measurement to the polishing work and therefore a drop in productivity of the magnetic disks would be induced.